1. Field of the Invention
The present invention relates to an imaging lens. In particular, the present invention relates to an imaging lens of a two-lens structure that is capable of reducing size and weight and enhancing optical performance. The imaging lens is used for an image-taking device that forms an image of an object, such as scenery and human figures, on an image-taking surface of a solid image pickup element such as a CCD, CMOS, etc. The solid image pickup element is mounted on a portable computer, a television phone, a portable phone, and the like.
2. Description of the Related Art
Recently, there has been an increasing demand for cameras that utilize a solid image pickup element, such as a CCD, CMOS, or the like, which is mounted on a portable phone, a portable computer, and a television phone, for example. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Further, in recent years, there has been an increasing demand for a high-optical-performance lens system capable of sufficiently utilizing resolution capabilities of a solid image pickup element having a high resolution exceeding one million pixels. Achieving a balance between size and weight reduction and improvement in optical performance is becoming increasingly important.
From this perspective, a two-lens structure lens system that is smaller and lighter than a three-lens structure lens system and superior to a single-lens structure lens system in optical performance is advantageous. Lens systems, such as those described in Patent Literatures 1 to 6, have been proposed as such two-lens structure lens systems.    [Patent Literature 1] Japanese Patent Unexamined Publication No. 2004-294829    [Patent Literature 2] Japanese Patent Publication No. 4071817    [Patent Literature 3] Japanese Patent Unexamined Publication No. 2004-109585((Especially, see FIRST and SECOND EXAMPLES)    [Patent Literature 4] Japanese Patent Unexamined Publication No. 2004-191844(Especially, see THIRD EXAMPLE)    [Patent Literature 5] Japanese Patent Unexamined Publication No. 2006-154517    [Patent Literature 6] Japanese Patent Unexamined Publication No. 2007-322478(Especially, see FIFTH EXAMPLE)